This invention relates to a unique sensing device, based upon the resistance-tape principle, which is known commercially and described in the literature under the trademark "Metritape" sensor, and which is the subject of several U.S. Pat. Nos., including 3,511,090; 3,583,221; and 3,792,407.
The Metritape resistance-tape sensor comprises an elongated metallic base strip having electrical insulation on the edges and back of the strip to define an uninsulated zone along the frontal length of the base strip, and a resistance wire, or ribbon, helically wound around the insulated base strip, with the helical turns bridging the insulated edge portions and being spaced away from the underlying uninsulated frontal zone of the base strip.
This sensor structure is enclosed within a continuous, flexible polymeric or other protective sleeve which provides a clean and dry inner chamber for the wound electrical element. The sensor is disposed vertically within a tank or vessel containing the liquid or fluent material, the level of which is to be monitored. The pressure of the material surrounding the immersed sensor causes deflection of the enclosing sleeve and this presses helical turns in the immersed portion of the sensor into engagement and electrical contact with the underlying base strip, such that an electrical resistance proportional to height of the voide space above the material is provided.
Applications for this elongated resistance-tape sensor have ranged from the gauging of short land-based tanks, to deep oil and ballast compartments on ocean-going supertankers; from tanks open to atmosphere, to closed heavy-walled tanks containing several atmospheres of pressure; from tanks never filled above 98% of level, to tanks that may be overfilled to heights that impart an overpressure upon tank and sensor components; from tanks containing water and benigh liquids, to those holding corrosive chemicals, aggressive solvents, and slurries and sewage having high solids content.
Within this wide application range, it is the role of the outer sensor envelope, or sheath, to isolate the critical inner electrical element from the outer surrounding material and from the vapors that may exist above the gauged material. In addition to resisting corrosion, the outer sheath must guard against the penetration of liquid and vapor by leakage, by seepage through pinholes and pores, and by permeation of liquid and vapor through the molecular structure of the one or more surrounding layers.
The molecular structure of all materials allows the penetration and permeation of gases and liquids to some degree, with the extent of such penetration being dependent upon physical properties of the material itself and upon its thickness. Layers of materials of elastomeric or polymeric nature, may allow liquid and vapor phases to pass through the structure to a limited degree and in either direction. The permeation of vapors and liquids through glass and metals, however, is extremely low, and thus thin metal foils, vacuum-deposited metallic particles, and elastrostatically-deposited glass particles are used as barriers to reduce the permeation of liquids and vapors through thin membranes.
While serving as an effective barrier, the outer protective layer, or layers, of a resistance-tape sensor must also remain compliant to serve as a flexing pressure-receiving diaphragm, yet be sufficiently rigid to bridge across helix turns, and deliver to them sufficient actuation force to bring them firmly into contact with the underlying base strip.
The outer cover of a resistance-tape sensor may take two general forms:
(1) A single composite sheath comprised of a multiplicity of thin film layers, held intimately together over their entire surfaces with solvent-resistant adhesive. This structure has only a single internal air cavity, and allows the use of special permeation barrier films as part of the composite jacket structure. However, the single multi-layer sheath is dependent upon long-term solvent-resistance of the adhesive material used to hold together the jacket layers; and this style of sensor has been subject to delamination by certain crude-oil components, gasoline additives and chemical solvents.
(2) An inner sensor having a first protective layer, inserted into a second outer sheath comprised of a formed thermoplastic material, which is heat sealed over its full length and at the bottom end, and is thus not dependent upon adhesives for the integrity of this outermost envelope. The present invention deals with important extension and refinement of this multi-wall form of sensor sheathing and its proper venting to surrounding atmosphere through a capillary breather/equalizer assembly.
Accordingly, it is an object of this invention to provide an improved outer protective sheath surrounding the electrical system of a resistance-tape material-level sensor.
Another object is to provide multiple barrier layers which cooperate to resist entry of corrosive liquid or vapor into the critical inner sensor chamber.
A further object is to provide multiple independent sheaths which operate together mechanically to deliver hydrostatic pressure efficiently to the discreet helix windings, bringing them into firm contact with the underlying conductive base strip.
Recognizing that all materials are penetrated to some degree by vapors and liquids, it is another object of this invention to provide internal and external sheathing layers that are relatively impervious to permeant vapors and condensate liquids.
A further object is to provide multiple internal chambers, each having venting means for pressure-equalization to surrounding atmosphere, and for the expulsion of permeant vapors by the pumping action of rising liquid level.
Another object of this invention is to utilize the low-permeation properties of glass and metallic materials as a means for reducing the permeation of vapors and liquids through the thin polymeric membranes used as inner or outer sheaths of resistance-tape sensors.
Yet another objective is to design an outer sheath for a resistance-tape sensor which is mechanically rugged and resistant to damage in the handling, transit and use of these sensors.